Locale: Support for Percent Formatting

Updates BNumberFormat to be able to format
percentages. Also re-introduces some unit
tests and updates the BNumberFormat ones.

This doesn't actually fix #16312 as the
defaults for percentage formatting don't seem
to track the selected language, but goes part
way there.

Related to #16312

Change-Id: Id6ddf426ce5571f4e8513c0eb1663cf42ac53cb1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3767
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

BMessageFormat: Rename to BStringFormat.

The ICU class is named MessageFormat, but on Haiku, it sounds too much
like something related to BMessage (which it isn't in the slightest)
and not part of the Locale system. It works almost entirely with BStrings,
so naming it BStringFormat makes much more sense.

OK'ed by PulkoMandy and Humdinger.

Add unit tests for BRelativeDateTimeFormat.

Signed-off-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Add BMessageFormat class.

This can be used to format complex messages properly. It moves the
complexity of handling plural forms, gender, and anything else needed
into the localizable string, rather than hardcoding it in the code.

This moves the difficulty of handling these things properly to people
doing translations, rather than relying on developers to do it.

Fixes #10755, but our localization must now be updated to make use of
the feature.

Add tests for BDurationFormat and BTimeUnitFormat.

Add tests for BDateFormat.

Convert language tests to cppunit.

Convert collator test to cppunit.

Convert BUnicodeChar tests to cppunit.

Add BMessageFormat class.

This can be used to format complex messages properly. It moves the
complexity of handling plural forms, gender, and anything else needed
into the localizable string, rather than hardcoding it in the code.

This moves the difficulty of handling these things properly to people
doing translations, rather than relying on developers to do it.

Fixes #10755, but our localization must now be updated to make use of
the feature.

Add tests for BDurationFormat and BTimeUnitFormat.

Add tests for BDateFormat.

Convert language tests to cppunit.

Convert collator test to cppunit.

Convert BUnicodeChar tests to cppunit.